The present invention relates to a polishing apparatus, which, in particular, is suitable for planarization of surfaces of work pieces such as semiconductor wafers for semiconductor integrated circuit devices with a multilayer interconnection comprising a plurality of metal films.
Recently, it is seen that planarization of surfaces of interconnect substrates for large scale semiconductor integrated circuits (referred to as xe2x80x9cLSIxe2x80x9ds hereafter) is important. One of representative techniques for the planalization is the Chemical Mechanical Polishing; CMP (referred to as xe2x80x9cpolishingxe2x80x9d as long as there is not any notification).
The polishing techniques are classified roughly into two processes:
a process utilizing a mechanical polishing property of abrasive grains; and
a process dominantly utilizing a chemical surface reaction effect with polishing with abrasive grains enhancing the reaction.
The former process is mainly used for planarization of insulator films such as silicon oxide (SiO2), alumina (Al2O3), or silicon nitride (SiN). An example, in which polishing of SiO2 is applied to semiconductor integrated circuit fabrication, is described, for example, xe2x80x9cPROCEEDINGS VLSI MULTILEVEL INTERCONNECT CONFERENCE 1991, A POUR-LEVEL-METAL FULLY PLANARIZED INTERCONNECT TECHNOLOGY FOR DENSE HIGH PERFORMANCE LOGIC AND SRAM APPLICATIONSxe2x80x9d 20-26. Concentration of abrasive grain in slurries for these polishing process is generally high, often ranging in 10-25 weight percent.
The latter process, in which chemical reaction mainly works, is mainly used for metal-film planarization, and described in detail, for example, in JP-A-2-278822 specification and JP-A-8-83780. Concentration of abrasive grain in the slurries is often 5 weight percent or less. A process, in which metal-films are polished in a liquid containing substantially no abrasive grain, is disclosed JP-A-11-195628.
It is assumed that an in-between process of the above two processes is polishing of (Si) substrate. Slurry for insulator is used in the substrate polishing, but it is thought that dominance of chemical reaction effect with respect to mechanical polishing effect in the substrate polishing process is greater than the one in SiO2 polishing process.
Additionally, processes to polish silicon wafers, glass substrates, or the like, in which instead of polymer-resin polishing pad, a polishing pad with fixed abrasive grains such as silica or cerium oxide (referred to as xe2x80x9cgrindstonexe2x80x9d hereafter) is used and slurry itself does not contain any abrasive grain, are disclosed in such as JP-A-10-125880 or JP-A-8-64562. A process to polish copper using similar grindstone is described in xe2x80x9cPROCEEDINGS SEMI TECHNOLOGY SYMPOSIUM 1998, A NEW SLURRY-FREE CMP TECHNIQUE FOR CU INTERCONNECTSxe2x80x9d 5-72 to 5-78. However consideration must be made, since principle of each of above methods using such fixed abrasive grains is similar to principle of polishing using slurry with respective abrasive grain and therefor the method easily generate polishing damage while having good planarizing effect.
It is necessary that polishing is performed uniformly over a predetermined area of an interconnect wafer when above mentioned polishing process is applied for planarization of surface of the interconnect wafer. In order that polishing is performed uniformly, it is needed that at least a surface of an interconnect wafer, which is to be polished, is pressed onto a polishing pad with uniform pressure. For uniform pressing, a variety of polishing apparatuses, particularly carrier structures to hold an interconnect substrate therein and mechanism for applying uniform pressure onto an interconnect substrate in the carriers, are being developed.
Fluid pressing mechanism is known as a carrier structure suitable for uniform pressing. As the fluid pressing mechanism, two type is known; a type of mechanism in which pressure is applied with air or liquid onto backside of an interconnect wafer (referred to as xe2x80x9cdirect fluid pressing mechanismxe2x80x9d hereafter), and a type of mechanism in which pressure is applied by pressing a flexible rubber-like sealed bag onto backside of an interconnect substrate (referred to as xe2x80x9cfluid bag mechanismxe2x80x9d).
The latter, as an example described in xe2x80x9cThe Japan Society for Precision Engineering, Autumn Conference 1991, conference paper publication, TRIAL MANUFACTURE AND BASIC CHARACTERISTICS OF POLISHING APPARATUSxe2x80x9d 211-212, has a structure wherein pressure applied onto a carrier is transmitted through a fluid bag constituted by a balloon-like membrane to an interconnect substrate and wherein an annular retainer surrounds an interconnect substrate to confine the substrate during polishing. Pressing onto an interconnect substrate is carried out by filling gas within the fluid bag. It is assumed that pressing with such fluid bag provides uniform pressing over backside of an interconnect substrate. In this example, the fluid bag is not secured to periphery of a substrate. Sealed container filled with fluid is pressurized by a weight. Since the weight or the fluid bag is not fixed, a guide is further provided outside of the retainer in order to prevent them coming off.
As described above, polishing apparatuses in which a weight or a fluid bag is not secured to a guide, are not suitable for polishing of many interconnect substrates since load and unload of an interconnect substrate are complicated. To solve the problem, a mechanism wherein a fluid bag is secured to a carrier is used recently.
The portion of a carrier that contacts with an interconnect substrate is provided with substantially planar surface, since once foreign materials infiltrate into the mechanism when polishing is performed they may generate polishing damage or contamination. For example, surface and inner surface of the retainer are finished with surface smoothness such that they have luster. A membrane composing the fluid bag is flexible and made of flexible rubber material with large friction such as neoprene or soft silicone. Therefor the membrane does not have luster like the retainer, but is finished with generally smooth surface. In the direct fluid pressing mechanism, rubber-like or polymer-resin-like layer with smooth surface is made contact only with periphery of backside of an interconnect substrate, and pressing is then carried out by increasing fluid pressure on the backside of the interconnect substrate while retaining them in sealed states or the similar states.
In all cases above mentioned for fluid pressing mechanism, torque for rotating an interconnect substrate is first applied to the carrier, then applied to the interconnect substrate through elastic material or thin film of rubber-like material or polymer resin. The interconnect substrate is thus supported flexibly, and a characteristic is provided that a substrate is secured to carrier or other transfer mechanism while allowing twist and eccentricity. Such mechanism in a polishing apparatus with fluid pressing mechanism consists of membrane composing flexible portion contacting with an interconnect substrate and a flexor that connects a carrier with the membrane.
As described above, for use in polishing of interconnect substrates for such as semiconductor integrated circuit devices, a variety of slurries that have not only mechanical effect but also surface chemical reaction effect, and polishing apparatuses comprising a carrier with fluid pressing mechanism such as fluid bags, have been developed.
However, a problem for unstable polishing arise in that polishing by using such polishing apparatus with fluid pressing mechanism and chemical-effect-dominant slurry often results in periodical high frequency sound (high frequency noise) generation and significantly lowered polishing rate. Such unstable polishing usually does not occur when slurry for insulator is used, but frequently occurs when slurry utilizing surface chemical reaction and having abrasive grain at concentration of 5 weight percent or less, i.e. slurry for metal containing substantially no abrasive grain, is used. When slurry substantially no abrasive grain is used, unstable polishing does not occur if a conventional polishing apparatus with pressing mechanism other than fluid pressing mechanism.
As described above, there is a problem that it is difficult to utilize in stable manner the combination of chemical-reaction-dominant slurry containing low or substantially no abrasive grain and causing little surface damage of interconnect substrates, and a polishing apparatus with fluid pressing mechanism providing good uniformity.
The present invention provides a polishing apparatus suitable for workpieces such as semiconductor wafers for semiconductor integrated circuit devices.
The present invention further provides a polishing apparatus for improving polishing performance and for reducing high frequency noise generation when polishing a workpiece such as semiconductor wafers in semiconductor integrated circuit devices manufacturing.
The present invention further provides a polishing apparatus for semiconductor integrated circuit devices for polishing a surface of metal film into stable and planarized state using slurry, or polishing solution (or polishing agent) containing substantially no abrasive grain.
The inventor first assumed that sliding occurs between a workpiece (referred to as xe2x80x9cwaferxe2x80x9d hereafter) and a membrane, causing wafer vibration during polishing. Based on the assumption, porous resin layer was inserted between the membrane and a wafer to increase the friction between them, but substantial improvement was not made. Additionally, based on an assumption that the too flexible membrane causes pressing unstable, membrane hardness was increased, but any substantial effect was obtained.
Regarding causes of such unstable polishing, the inventors noticed the fact that polishing is stably performed with mechanical-effect-dominant slurry containing high concentration of abrasive grain even if the fluid pressing mechanism is used. Therefore, to slurry containing substantially no abrasive grain, described in JP-A-11-195628, alumina powder was added as abrasive grain such that the mixture had concentration of 10 weight percent of alumina. The polishing was then applied copper film on a wafer surface using the 10 percent mixture. It was then confirmed that above mentioned unstable polishing did not occur.
The present invention is directed to slurry with concentration of abrasive grain of 5 percent or less, and polishing solution (or polishing agent) to which no abrasive grain is intentionally added. There are many differences in composition and feature between above two substances, but both have same behavior regarding the object of the present invention. In this sense, both will be referred to as xe2x80x9cslurry containing substantially no abrasive grainxe2x80x9d, hereafter.
Based on those characteristics, the inventor found that high frequency noise is generated through mechanism described below, and came up with a measure to reduce the noise from a viewpoint of the mechanism, then verified that the measure is effective.
When slurry with high concentration of slurry in a polishing apparatus with fluid pressing mechanism is used, a fluid bag first (hence a membrane,) expands for pressing. Since a polishing pad is moving relatively with respect to the wafer (,hence providing relative motion,) during polishing, a membrane side of the fluid bag in the carrier contacts with inner wall of the retainer at the most downstream portion of the relative motion. Because the fluid bag has certain degree of freedom with respect to rotation of entire carrier, and friction between the wafer and a surface of the polishing pad on a polishing platen is large, the fluid bag does not necessarily follow the rotation of the carrier or retainer completely, and it may cause twist of the membrane or flexor, or eccentricity of the membrane. However, since abrasive grains infiltrated between membrane and retainer are sandwiched between the membrane side and the retainer, the contacting between them is maintained stably, and the fluid bag continues to rotate with the twist with certain delay with respect to the carrier.
As a contrast, when slurry containing abrasive grain in low concentration of 5 weight percent or less, or substantially no abrasive grain is used, the effect of abrasive grain is not sufficient so that side of the membrane contacts with inner wall of the retainer at the most downstream potion of the relative motion on start of polishing. When slurry contains abrasive grain as well surfactant, even though it contains abrasive grain, the side of membrane sticks tightly with the retainer, causing the membrane and a wafer to rotate following rotation of the carrier and retainer.
The force needed to break the sticking state (referred to as xe2x80x9csticking forcexe2x80x9d hereafter) is a power generated by the relative motion between a wafer and the polishing pad. In other hand, a predetermined force is needed to rotate a wafer through carrier, and is referred to as xe2x80x9crotational frictionxe2x80x9d. In the case of slurry with low concentration of abrasive grain, rotational friction is less than sticking force. However, as the carrier rotation is continued, thus the sticking portion moves laterally or upstream from the most downstream, sticking force between the retainer and membrane is reduced, and the sticking state is broken at the moment when sticking force become less than rotational friction, then new sticking portion is generated at the most downstream portion of the relative motion.
In repeating such separation of sticking portion or generation of new sticking portion, high frequency noise is generated due to friction or vibration between side of the membrane and a inner wall of the retainer. Such vibration lowers sticking firmness between a wafer and the polishing pad, reducing polishing rate. The inventers found those facts.
The inventers also found that in order to prevent such generation and breakup of sticking potion, and generation of new sticking portion, methods described below is effective.
In the specification, since a carrier comprising a case (box body) to which an elastic membrane secured, and retainer (wafer receptor) attached to bottom of the case covers side and a backside of a wafer placed on a polishing platen, the carrier is referred to as xe2x80x9ccover bodyxe2x80x9d. Additionally, the carrier is referred to as xe2x80x9cwafer holderxe2x80x9d, since the carrier hold a wafer over the polishing platen during polishing with fluid pressure within the carrier.
The inventor found that stable polishing is provided without high frequency noise generation by using polishing apparatus employing one or combination of following three methods. The first method is such that sticking force between the membrane and the retainer (a receptor of a workpiece like a wafer) is reduced to less than rotational friction. The second method is such that the retainer is made rotatable with respect to the case (box body) of the carrier (cover body or holder for a wafer) so that sticking portion between the retainer and the membrane does not move. The third method is such that flexor strength is increased so that the membrane does not easily stick to the retainer.
Specifically, in the first method, by composing inner wall of the retainer, which contacts at least with the membrane, of material to reduce the sticking force between the membrane and the retainer to less than the rotational friction, it is ensured that sticking portion is always at the most downstream portion. Fluorocarbon resins such as tetrafluoroethylene or trifluoroethylene, are suitable for such material.
Another method to lower the sticking force is to provide grooves or height variation in surface of the inner wall of the retainer such that the retainer does not easily sticks to the membrane. It is confirmed that depth and pitch of the groove or height variation is preferably larger than size of abrasive grain size of used slurry, and size larger than 10 micron provides practical and stable lowerd sticking force. By provision of plurality of such grooves in longitudinal or lateral direction, abrasive grain or slurry is actively retained in the groves.
A still further method is to lower sticking force by coating fluororesin in thickness of 10 micron to 100 micron onto side of membrane opposing to inner wall of the retainer, or by providing side of the membrane with grooves or height variation. Combination of them can further reduce sticking force and is thus effective in prevention of unstable polishing.
In any of above methods, pressure of fluid is controlled, the fluid is introduced to expand the membrane such that a wafer rotates while the side of the membrane is pressed onto the inner wall of the retainer to contact therewith.
In the second method, a retainer has a structure to be rotatable with respect to the case of the carrier so that the retainer and membrane always can rotate together and sticking portion therebetween is retained stably at the most downstream of the relative motion without undesired friction. The method is in principle most suitable for prevention of unstable polishing. However, it preferably use a structure such that it can decrease possibility that abrasive grains or foreign particle infiltrate into the mechanism which allows the retainer rotate with respect to the carrier case, generating new particles, then generating polishing damage.
In the third method, strength of a flexor, which is an elastic fixing member for the membrane, is increased so that sticking between a membrane and a retainer, or migration of sticking potion does not easily occur that would otherwise occur due to twist. As a flexor, thin film of rubber or polymer resin in thickness of 0.5 mm or less may be used, however it is preferable that the thickness is more than 0.5 mm and that the effective strength is increased more than two times by using harder material. Thin plate made of stainless steel or phosphor bronze that has good elasticity, or hard resin with high wear resistance such as polyurethane resin, fluorine-contained resin, silicone resin, or nylon resin, is suitable for the hard material. Since this method allows membrane to move upward and downward with respect to the retainer, the membrane does not easily twist with respect to the entire carrier, and not easily stick to the retainer due to deformation. However, a caution should be made that too high flexor strength lowers polishing uniformity.
By using one of above described methods, or any combination of them, generation of high frequency noise and unstable polishing can be prevented. Particularly, it is significantly effective in polishing of metal film with slurry containing substantially no abrasive grain.